Spectroscopy is an effective tool for identifying and characterizing a vast array of substances. Many spectroscopic techniques, instruments, and methods are well known. For example, in absorption spectroscopy a characteristic of a component in a sample, such as its presence or concentration, may be determined by evaluating an absorption characteristic of a light beam passing through a sample. The absorption characteristic is typically measured by generating a signal representing a comparison of the beam intensity as it enters and exits the sample, at one or more spectral regions of interest. It is well known that the entering and exiting intensities are typically related by Beer's law in which the ratio of exiting to entering intensity depends on the inverse of the exponent of the path length. That is, the greater the path length the greater the measured absorption signal, other factors remaining the same, up until a path length is reached where all light of the spectral region of interest is absorbed (the signal saturation point). After signal saturation no further change in absorption signal is possible even as the concentration of a species of interest in the sample increases. So it is useful to have longer path lengths to increase the absorption signal and thereby increase the signal to noise ratio from an instrument when the species of interest has a low concentration, but to allow for shorter path lengths to accommodate samples with higher concentrations of the species of interest without signal saturation.
Other forms of spectroscopy include Raman and fluorescence spectroscopy where a characteristic of a species of interest in response to a light beam are measured, but which characteristic is not simply an absorption characteristic of a light beam. For example, in Raman spectroscopy, light typically from a laser and of a known wavelength (typically infrared or near infrared) is directed at a specimen. The laser light (also sometimes referred to as the Raman pump) interacts with the electronic states in the molecules of the specimen and, as a result of this interaction, experiences selected wavelength shifting. The precise nature of this wavelength shifting depends upon the materials present in the specimen. A unique wavelength signature (typically called the Raman signature) is produced by each specimen. This unique Raman signature permits the specimen to be identified and characterized. While the absorption of the incident light beam is not measured directly, the measured signal in Raman spectroscopy also depends on the volume of sample observed which interacts with the incident beam. Thus, path length can also be important in Raman spectroscopy, fluorescence spectroscopy, and other forms of spectroscopy.
Spectrometers with very long sample cells can be conceived. However, very long sample cells can occupy too much space on a laboratory instrument and are particularly impractical on portable instruments. Herriott cells and White cells are well known as solutions to provide longer path lengths. In a Herriott cell, two mirrors face each other from opposite ends of an elongated cell and incident light enters through one mirror, and is reflected back and forth multiple times before exiting through one of the mirrors. A White cell uses three mirrors instead of two, and is able to accept high numerical aperture (NA) light. Such cells are still rectangular in shape and can be inconvenient in size, and are still of a fixed length.